Heretofore, several devices and methods have been proposed that act on charged particles (ions) in a multi-species plasma for the purpose of separating particles of different mass/charge ratios from each other. In particular, these devices have been designed and engineered to use crossed electric and magnetic fields to effect charged particles in a plasma. For example, U.S. Pat. No. 6,096,220 which issued to Ohkawa for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention, discloses a device and method for separating charged particles of a multi-species plasma in a plasma chamber. In accordance with this invention, an axially oriented magnetic field is crossed with a radially oriented electric field in a manner that causes particles having mass/charge ratios above a predetermined cut-off mass (Mc) to follow unconfined orbits. Consequently, these particles are collected inside the filter chamber. On the other hand, particles having mass/charge ratios below the predetermined cut-off mass (Mc) are confined on orbits that cause them to exit the chamber for collection. A variation of the above-mentioned invention disclosed in U.S. Pat. No. 6,719,909, which was issued to Ohkawa on Apr. 13, 2004, for an invention entitled “Band Gap Plasma Mass Filter,” and which is also assigned to the same assignee as the present invention, employs a device and method for tuning the radial electric field with a sinusoidal component. This tuning then causes the crossed electric and magnetic fields to target particles of a predetermined mass/charge ratio for confinement in the filter chamber, rather than relying on a demarcation above and below a cut-off mass. As indicated, in these examples, the respective magnetic fields are axially aligned and the respective electric fields are radially oriented. Further, the radial electric fields of these inventions are generated by electrodes.
Depending on the particular application, it is well known that when electrodes are used to generate electric fields, the electrodes can adversely affect their environment if they are not properly controlled. In this respect, plasma mass filters that employ electrodes to generate radial electric fields are no exception. The import here is that the physics and engineering issues implicated in such applications need to be considered. On the other hand, if electrodes are not used to generate an electric field and, instead, the electric field can be induced by other means, the adverse issues alluded to above are generally obviated.
In accordance with basic physics, it is well known that a moving magnetic field can be used to induce an electric field. With this in mind, and by using appropriate assumptions for conditions inside the chamber of a plasma mass filter, it can be mathematically shown that the sinusoidal component of an axially oriented magnetic field will induce an azimuthal electric field Eθ. For this purpose, the magnetic field can be generally defined by the expression Bz=B0+B1 sin ωt. Further, when a plasma filter is operating near the Alfven cavity mode, or when there is a low plasma density in the filter chamber, additional appropriate assumptions allow an α-β plot (α is abscissa and β is ordinate) to be mathematically established. Specifically, such an α-β plot can be used to determine the operational parameters that will define whether a charged particle, having a selected mass/charge ratio (M), will travel on a confined or and unconfined orbit in the separation section of the plasma chamber. For the α-β plot,       α    =                            Ω          0          2                +                              Ω            1            2                    /          2                            4        ⁢                  ω          2                      ,            and      ⁢                           ⁢      β        =                            Ω          0                ⁢                  Ω          1                            8        ⁢                  ω          2                      ,where Ω0 is the cyclotron frequency for particles with mass/charge ratio M, and wherein Ω0=B0/M and Ω1=B1/M.
In light of the above, it is an object of the present invention to provide a band gap mass filter using an azimuthal electric field (Eθ) to separate particles of mass (M1) from particles of mass (M2) in a multi-species plasma which effectively confines the electric field inside the separation section of the filter. Another object of the present invention is to provide a band gap mass filter that effectively obviates the adverse effects that would otherwise result if electrodes were used to generate the electric field. Still another object of the present invention is to minimize the in-vessel components of a band gap mass filter. Yet another object of the present invention is to provide a band gap mass filter that is relatively easy to manufacture, is simple to use and is relatively cost effective.